The acute effect of marijuana on CBF and relevant variables was examined in 35 normal male volunteers. Subjects came to the laboratory 3 times, visits were separated by a minimum of 1 week. During each visit, CBF was measured once before and 3 times after (30 minutes, 60 minutes and 120 minutes) smoking a high-potency marijuana cigarette (THC = 3.55%), low-potency marijuana cigarette (THC = 1.75%) and placebo cigarette on a double-blind basis. CBF showed significant increase which peaked 30 min after marijuana; no changes were seen after placebo. Post-marijuana CBF increase was most marked in the frontal lobe and in the right hemisphere. CBF time course did not parallel that of THC plasma levels which peaked around 5 min after commencement of smoking and came down substantially within the first 15 minutes. After marijuana, pulse rate and respiratory rate increased significantly. End tidal carbon monoxide increased but not carbon dioxide levels. Forehead skin blood flow did not show appreciable changes. Self- ratings of intoxication peaked after 30 minutes of marijuana smoking. State anxiety showed a trend towards increase after marijuana while somatic anxiety showed a highly significant increase. Temporal Disintegration Inventory and Depersonalization Inventory showed highly significant increases after marijuana. Stepwise multiple regression analysis identified plasma levels of THC as the only significant physiological/biochemical predictor of post-marijuana CBF. Rating scales proved to be more significant predictors of CBF change in a combined model which included rating scales and physiological indices.